1. Field of the Invention
This invention relates to a lead frame and, more particularly to a multi-layer lead frame having at least one metal plate or plane, hereinafter referred to as "metal plane" and used for a semiconductor device. This invention also relates to a semiconductor device using such a multi-layer lead frame.
2. Description of the Related Art
A conventionally known multi-layer lead frame for a semiconductor device comprises at least one metal plate or plane connected via an insulative layer to the bottom surfaces of inner leads. The use of such a multi-layer lead frame enables the mounting of a relatively power-consumable semiconductor chip on the metal plane, due to a heat radiation of the metal plane. Also, the metal plane can be used as a ground layer or a power supply layer, to thereby improve the electrical characteristics of the semiconductor device.
FIG. 4 shows a conventionally known semiconductor device using such a multi-layer lead frame. In this example, two metal planes, i.e., a power supply metal plane 12 located under the inner leads 10 and a ground metal plane 14 located under the power supply metal plane 12, are used. These metal plane 12 and 14 are fixed to the inner leads 10 and the plane 12, respectively, by insulative layers 16 and 16a, respectively, made of, for example, a polyimide. The power supply metal plane 12 and the ground metal plane 14 are electrically connected to power supply lines and ground lines, respectively, of the inner leads. The lead frame including the inner leads 10 and metal plane 12 and 14, and a semiconductor chip 18 mounted on the ground metal plane 14, are hermetically sealed or molded with a resin 30.
In such a resin molded type semiconductor device, however, if the temperature of the device becomes to high, the molded resin is easily peeled from a stage portion of the metal plane on which a semiconductor chip is mounted, and cracks might appear in the molded resin. To prevent the generation of such cracks in the molded resin, the stage portion is provided beforehand with many dimples, notches, or through holes, to thereby improve the adhesion between the molded resin and the stage portion. Alternatively, the stage portion is provided with slits or recesses to prevent an ingress water or the like to the molded resin, which might come into contact with the semiconductor chip. In a multi-layer lead frame, however, since the stage (in this case, the metal plate) is much larger than a stage portion of usual flat type or single-layer lead frame, the water component contained in the molded resin might be expanded when the semiconductor device is subjected to a soldering process or when the semiconductor chip generates heat, and such an expansion of the water component in the molded resin might cause stress therein, and cause cracks to appear in the molded resin.
In addition, in a multi-layer lead frame, since the metal plane is fixed to the inner leads by an insulative layer 16, to effect an electrical insulation therebetween, an adhesive material, such as a polyimide, used as the insulative layer 16 is generally hygroscopic and, therefore, easily causes cracks to appear in the molded resin.
Particularly, the coefficient of thermal expansion of the resin is generally larger than that of the metal plane, and therefore, the molded resin might be peeled from the metal plane (in particular, a central portion of the metal plane), under a high temperature condition, due to a difference in the coefficients of thermal expansion of the resin and metal plane.
Also, because a wire-bonding onto the metal plane usually occurs in the vicinity of the semiconductor chip, a stress to which the wire-bonding area of the metal plane is subjected is larger than that at a wire-bonding area of the top surfaces of the inner leads. Therefore, a strength of the bonding connection might be weakened, and thus the bonded-wire easily peeled from the metal plane. Therefore, if the bonding-area is repeatedly subjected to stress, the molded resin might be peeled from the metal plane and the bonded-wire might be also easily peeled from, and therefore disconnected from, the metal plane.